Transformer core construction



Oct. 1, 1968 R. v. WOODWARD TRANSFORMER CORE CONSTRUCTION Filed Oct. 22,1965 INVENTOR REX V. WOODWARD ATTORNEYS United States Patent 3,404,360TRANSFORMER CORE CONSTRUCTION Rex V. Woodward, Mount Vernon, Ill.,assignor to Power Cores, Inc., Mount Vernon, 11]., a corporation ofIllinois Filed Oct. 22, 1965, Ser. No. 501,058 9 Claims. (Cl. 336-211)ABSTRACT OF THE DISCLOSURE A laminated transformer core in which thebutt-joints of all laminations in one of the core legs are staggeredrelative to each other and distributed over the entire length of the legexclusive of the stressed regions immediately adjacent the corners ofthe core.

The present invention relates to transformer cores and the assembly andfabrication thereof. In particular, the present invention relates to aformed, or wound, transformer core having improved magneticcharacteristics.

One type of transformer construction, well known in the art, provides apreformed coil element having a center portion which forms a windowtherein. A magnetic core passes through the coil window and encloses aportion of the coil, forming a magnetic circuit thereabout. Suchcoresare usually formed by the juxtapositioning of a plurality of laminationsinto a plurality of separate groups. Each group of laminations isgenerally of rectangular or square shape which consequently defines thegeometric configuration of the core.

In the manufacture of such cores, each lamination is cut to theappropriate length, which decreases for each lamination, from the outerone to the inner one, corresponding to the respective decreases in therequired perimeter. The laminae are then formed into annular groups, bymethods known in the art, and each group is concentrically nested, onewithin another. Each lamination is butt jointed, the particular locationof each butt joint being of material significance with respect to thepresent invention, the details of which are discussed further on in thespecification.

The concentric structure is then shaped and heat treated to relievestresses in the core material produced by the forming operation.

Each group of laminations is separated from the structure and thenindividually spread .and inserted about a portion of the preformed coil,through the window thereof. The group having the smallest perimeter isinserted, first, the next larger superposed over the first, and so onuntil the outer group is spread and inserted about all of the others.Each group is closed with a butt joint at various locations on the core.It is known to locate such joints in various positions on the core withrespect to the coil window, for example, such as that shown by Hurt,Patent No. 2,702,936; Ellis, Patent No. 2,973,494 and Patent No.3,154,758. However, it has been found that a particular arrangement inaccordance with the present invention, yields improvements notheretofore achieved.

The nature of the formed core construction has presented numerousproblems in connection therewith, as for example, the magneticeificiency of such cores may suffer because perfect abutment of thelaminations is not generally possible, and, the resulting air gapsproduce a higher core reluctance than it perfect abutment wereachievable. In addition, the spreading and bending of the variouslaminations in fitting them to the coil creates internal stresses in thecore material which tend to increase'the magnetic core losses further.

In the fabrication of such cores it has been common practice to provideall of the butt-joints in the leg of the core which is disposed withinthe coil window. In so 3,404,360 Patented Oct. 1, 1968 doing, thepositioning of each joint and the retention of the various laminationsin the proper position during assembly is somewhat difficult. The groupof laminations last formed within the concentric core structure is separated therefrom and used as the first core group to be fitted to thecoil. Thus, this group functions as a core form about which theremainder of the laminations are assembled. The proper alignment of thebutt joints of this first group is thus critical because of its effecton the alignment of the remainder of the groups. Since it is verydifficult, if not impossible, to view the positioning of the jointswithin the coil window, the aforementioned problem is created.

In accordance with the present invention, the first group of laminationsis fitted to the coil in such a manner that the butt joints thereof arepositioned on a core leg disposed outside of the coil window. Hence, thejoints may be easily located and positioned properly. Thereafter, theremainder of the groups can be superposed over this first group suchthat the joints associated therewith will be positioned on the core legwhich is disposed within the coil window. The final and outer laminationis then reversed in the manner of the first group to provide an exposedbutt joint for easy bonding.

An additional and equally, if not more, important feature of the presentinvention, is that when the core is structurally arranged as indicatedabove, the performance of the core is measurably improved over the priorconstruction. This is contrary to that which would ordinarily beexpected by those skilled in the art, hence, the aforementionedconvention of positioning all of such joints within the coil window inthe path of the greatest flux density even though such a structurecomplicates the assembly and increases cost of the device.

In accordance with another feature of the present invention, it has beenfound that the most optimum core performance is achieved by positioningthose joints located on the leg within the coil window such that theyare arranged in a Zigzag distribution and widely and randomly spreadacross the full length of the leg. Each group has a random number oflaminations varying between general limits and no specific number isrequired.

Accordingly, it is an object of the present invention to provide aformed-type induction core and a method of fabrication therefor whereinsaid core has improved magnetic characteristics as compared with coresheretofore known.

It is another object of the present invention to provide a formed-type,butt-jointed, multigroup induction core having the butt joints of theinnermost groups disposed external to the coil window to provideimproved magnetic characteristics as well as easy alignment of thelaminations.

It is still another object of the present invention to provide aformed-type core with improved magnetic characteristics by positioningthe butt-joints of the laminations on the leg within the coil windowsuch that the joints form a zigzag distribution which is randomly spreadacross the entire length of the leg.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 illustrates the arrangement of the laminae in accordance withone embodiment of the invention.

FIGURE 2 illustrates, in an expanded view, one manner of arranging thevarious lamination groups in accordance with the invention and a fixturetherefor.

FIGURE 3 is a diagrammatic illustration of one method of forming thecore.

3 FIGURE 4 is a cross-sectional view of a magnetic structure during thefinal stage of assembly.

Referring now to FIGURE 1, there is shown one specific embodiment of theformed core in accordance with the present invention. Leg 16 of the coreis intended to be disposed within the window of a preformed coil, whileleg 17 is to be disposed external to the coil. The various laminae ofleg 16 are shown to be arranged in a zigzag fashion, having the buttjoints 12, 13, 14 and 15 spread throughout the entire permissible lengthof the leg. The butt joints which form each of the diagonals, representthe ends of the laminae of each group, respectively. The number oflaminae per group may be any random number, but preferably is a numberbetween 12 and 18. No particular overlap is essential, the amount ofoverlap would thus depend on the specific design criteria desired.However, it is essential that the joints of each group be spread acrosssubstantially the entire permissible length of the leg 16.

The expression entire permissible length of the leg" exludes thoseportions of each leg which are stressed due to the bends at the corners.These stress regions must be avoided. The length of the regionsdetermined by the thickness of the laminations which often are afunction of core size. As an example, in a core of about 7.4" x 4.7",weighing approximately twenty-two pounds, the exclusion region extendsfor about to from the corner. Other than for observance of thisrestriction, the joints may be spread generally stepwise in each groupalong the entire length of the leg.

The first group of laminations which contains a relatively small numberof laminae, preferably three, is shown having the stepped butt-joints10. These joints are positioned on the leg 17 external to the coil andpreferably are substantially centered, although this is not essential.

The last, or outer lamination is likewise arranged such that the buttjoint 11 thereof is also positioned on leg 17 and preferably centrallydisposed.

The method of assembling the core, in accordance with the invention, isillustrated in FIGURES 2 and 3. In particular, referring to FIGURE 2,lamination 23 is formed into a generally circular or oval shape andplaced between supporting surfaces 21 and 22. Surfaces 21 and 22 form asupporting jig for merely retaining the laminae in the required shapeand relationship during this part of the assembly procedure and suchfunction might be performed by other means known to the art. Thelamination 23 is positioned such that the butt joint 11 is locatedadjacent the surface 22. Next, the group of laminae 24 is nested withinthe lamination 23 such that the natural resilient tendency to uncoilretains it in position adjacent thereto, and not as shown in FIGURE 2,which is in expanded view only for the purpose of illustration. Thebutt-joints 12 of group 24 may he stepped in any manner known to the artand are arranged generally diametrically opposite to joint 11. Theremainder of the groups, such as 25, are then nested concentricallywithin one another, the last group to be inserted being group 26.

Group 26, containing relatively few laminae, as compared with the othergroups, is nested therein having the butt-joints 10 reversed in similarorientation to butt-joint 11 of lamination 23. The structure may betaped or held together in any conventional manner. This ultimatelyreduces the air gaps between the ends of the butt jointed laminae ofeach group. The arrangement of the various laminae in the same relativepositions as intended-in the final device structure, reduces the finalspace factor and thus, the resulting magnetic losses. The frictionbetween each lamination and the ones adjacent thereto, and the frictionbetween the lamination 23 and the surfaces 21 and 22, retain all thelaminae in the given configuration.

The number of groups of laminae used' in the core construction dependson the ultimate design of the inductive device and is not limited byconstruction in accordance with the invention.

Each core group of FIGURE 2 has its joints offset in a directionopposite to that of its adjacent groups whereby they form zigzagdiagonals extending over a sufficient length of the circumferencethereof to ultimately provide the disposition of the stepped joints oversubstantially the entire length of the leg 16 as shown in FIGURE 1.Thus, the vertex formed by each adjacent pair of diagonals would beapproximately in line in a direction transverse to the generallyparallel perimeters of each group.

The core is then formed to the desired shape by appropriate apparatus,as for example, by such as is dia grammatically illustrated in FIGURE 3.Specifically, there is shown a mandrel 32 which will determine theinternal shape of the core. The unfinished core structure 33 is placedon mandrel 32 and a force is applied to the ultimate leg portions by aram 36 and forming plate 31. The relative distances of the formingplates 35 serve to determine the final shape of the core. As the ram 36moves downward, the core portion directly therebeneath and the portionopposite thereto, between the mandrel 32 and the forming plate 31,straightens into a linear geometry, while the yoke portions of the corestructure abut against the forming plates 35 to result in a generallyrectangular core configuration. It should be noted that the plates 35must not be spaced too close together as to decrease the space factor inthe leg portions. Although such apparatus as is shown in FIGURE 3 may beutilized to produce the final shape of the core, other apparatus andfixtures might be used, the particular one of which forms no part of thepresent invention.

Once the core is formed, it is then subjected to a heating operation,for example, by means of an oven, in order to relieve the internalstresses locked in the core material. Thus, the core sets with theprescribed configuration and has substantially lower magnetic lossesafter such treatment. The core is then disassembled into individual coregroups after removing any ties or other binding or retaining meanswhich, of course, may have burned off during the heating operation. Theindividual core groups are now assembled about a preformed coil.

As shown in the cross-section of FIGURE 4, a coil is provided with twoleg portions 41 defining the coil window therebetween. The innermostcore group 26, of FIGURE 2, is individually separated from the structureand expanded to fit about the coil leg 41 in such a manner that the endsof the laminations abut one another externally of the coil window andare easily viewable by the assembler. This first, or innermost coregroup (illustrated as 42 in FIGURE 4) then serves as a form forassembling the remainder of the groups about it. The group 42, whichpreferably contains only three laminations, is easily arranged into theproper alignment and the joints are centered with respect to the coil.The shape of the group 42 now permits the remainder of the groups to bereadily aligned as group 43 is fitted onto group 42, and group 44 isfitted onto group 43, etc. A single lamination 45 is then fitted ontothe outermost group and arranged such that the butt-joint 11 is disposedopposite to the joint 10 of group 42. The joint 11 may then bespot-welded, for example, to permanently secure the core groups into aunitary structure. Likewise, the same procedure is used to form the corestructure on the opposite leg of the coil.

Thus, it has been found that by positioning the buttjoints along theentire length of the core leg within the coil window, in the path ofhighest flux density, improved performance in 'both lower watts and voltamperes per pound of core material is obtained as compared with priorart formed cores having the stepped butt-joints concentrated in a givenregion. Furthermore, by positioning the joints of the innermost threelaminations external to coil window, a further increase in performanceis obtained. The comparison of performance is illustrated by the data ofthe table below.

TABLE I VA per lb. Watts per lb. B

1s,340. IIIIIIIII'fii IIIIIIIl f The column indicated as B representsvarious values of flux density in gauss developed in the core duringtest. The next three columns represent the volt-ampere loss for threedifferent core constructions, types I, II and III, respectively. Thetype III construction is that shown in FIGURE 1 having the butt-jointsof the first three laminations external of the coil Window and with thejoints spread across the maximum permissible length of the leg asindicated hereinabove. The type II construction provides the butt-jointsof the first three laminations internal of the coil window in a mannersimilar to that of prior art core groups but again with the jointsspread across the entire core leg. The type 1 construction is that ofthe prior art with all joints within the core windows and the jointsgrouped together over a short length of the leg. The next adjacentcolumns list the corresponding values of the power loss in watts perpound of core material.

The cores tested each weighed approximately 22 pounds. The window sizewas 4% x 2", the legs and yokes were 1.34 inches thick and 3% incheswide.

The data presented in Table 1 is test data and it can be observed thatat all values of flux both the types It and III cores provide superiorresults. Thus a marked improvement is obtained as a result of thespreading of the joints over the entire permissible length of the legwithin the core window as opposed to concentrating the joints in a givenlocation.

The comparison between core types 11 and III also clearly illustratesthat the type III core is superior to the type II in the watts per poundcategory at all values of flux except at 15,833 gausses and the lastentry in each column. The deviation at a flux of 15,833 gausses cannotbe explained and may be due to faulty reading or achieving a higher fluxdensity than indicated. At the point of highest flux, the same fluxreading was not obtained for any of the three cores. However, theperformance of types :II and III cores at the upper end of theiroperating range is comparable.

As to the performance of the three cores relative to voltamperes perpound, types II and III are again clearly superior and type III issuperior to type '11 over all but the very upper end of the range.

The above analysis and data clearly indicates that distribution of thecore joints in an extended length of the leg in the coil window producessuperior results over concentration of the joints in one area oranother. Further, the location of the inner and outer laminationsexternally of the coil window produces better operation than when alljoints are located within the window. Although the increase inperformance of type :III over type II is not as significant as type IIrelative to type I, the increased ease of assembly is an importantfactor in favor of use of the type III core.

While I have described and illustrated one specific em bodiment of myinvention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

I claim:

1. A magnetic core comprising a plurality of generallyrectangular-shaped, butt-jointed laminations of high permeabilitymaterial, each said lamination comprising a closed loop having a singlejoint therein,

the rectangular shape of said laminations defining laminations havingtwo leg members and two yoke members joined by corners, the formation ofsaid corners creating internal stresses in said high permeabilitymaterial in regions of the material encompassing said corners andmaterial immediately adjacent said corners whereby the permeability ofsaid material in said regions is reduced relative to the remainder ofthe material of said laminations,

said laminations being of increasing size and nested one within theother to form a stack of contiguous laminations providing a core havingtwo leg members and two yoke members, and

a majority of said butt joints being located in one of said legs of saidcore, the butt-joints of all of said laminations being staggeredrelative to one another and distributed over the entitre length of saidleg exclusive of said regions whereby each butt-joint is disposedbetween continuous lengths of two adjacent laminations in which lengthsthe permeability of the material has not been reduced due to formationof said corners.

2. The combination according to claim 1 wherein the ratio of the lengthof the legs of said laminations to the length of said region at one endof said leg lies in a range of approximately 6 to 1 to 7 to 1 and higheras the length of the leg increases.

3. A magnetic core comprising a plurality of generallyrectangular-shaped, butt-jointed laminations of high permeabilitymaterial, each said lamination comprising a closed loop having at leastone joint therein;

the rectangular shape of said laminations defining laminations havingtwo leg members and two yoke members joined by corners, the formation ofsaid corners creating internal stresses in said high permeabilitymaterial in regions of the material encompassing said corners andmaterial immediately adjacent said corners whereby the permeability ofsaid material in said regions is reduced relative to the remainder ofthe material of said laminations,

said laminations being of increasing size and nested one within theother to form a stack of contiguous laminations providing a core havingtwo leg members and two yoke members,

a majority of said butt joints being located in one of said legs of saidcore, the butt-joints of all of said laminations located in said one ofsaid legs being staggered relative to one another and distributed overthe entire length of said leg exclusive of said regions whereby eachbutt joint is disposed between continuous portions of two adjacentlaminations in which portions the permeability of the material issubstantially unaffected by internal stresses created during formationof said corners.

4. The combination according to claim 3 wherein the joints of the threeinnermost laminations are disposed in another of said legs of said coreand the remainder of said joints are disposed in said one of said legsof said core.

5. The combination according to claim 4 wherein the joints in said oneleg are randomly disposed relative to one another.

6. The combination according to claim 5 wherein the ratio of the lengthof the legs of said laminations to the length of said region at one endof said leg lies in a range of approximately 6 to 1 to 7 to l and higheras the length of the leg increases.

7. The magnetic core according to claim 4 further comprising anoutermost lamination having a butt-joint positioned in said other leg.

8. The method of making a laminated magnetic core comprising the stepsof forming several groups of core members by cutting a plurality oflaminations of magnetic strip material of increasing lengths such thatthe ends of each lamination abut when formed into a closedconfiguration, said laminations and said groups being of such lengths asto tightly nest one within the other; arranging the butt-joints of thelaminae of each group in an oifset manner and extending over a lengthsuch that the joints extend over the entire length of a leg of the finalconfiguration exclusive of regions'encompassing the corners of the finalconfiguration and the adjacent areas having reduced permeability due tostresses created in the formation of the corners of the core; forming asingle lamination into a generally elliptical configuration having abutt-joint in a given position along the length thereof; clamping saidsingle lamination so as to retain it in said configuration and position;assembling the longest of said groups into a similar ellipticalconfiguration and inserting said longest group within said singlelamination with said offset buttjoints positioned opposite to thebutt-joint of said single lamination; assembling the successivelysmaller groups into elliptical configurations and inserting them insideof said longest group such that the butt-joints of said groups form azigzag pattern over a sufiicient length so as to result in said patternextending over the entire length of a leg in the finished core;assembling the innermost group into an elliptical configuration andinserting said group inside of the other groups such that thebutt-joints of said innermost group are positioned opposite to thebutt-joints of said other groups; pressing the assembly to its finalrectangular size and shape whereby corners are formed between the legsand the yokes of the core and heat treating said core to relieve strainsand to set the final configuration.

9. The method according to claim 8 further comprising the steps ofremoving said innermost group, spreading said group at the butt-jointsthereof, inserting said group through the window of a preformed coilsuch that said buttjoints are positioned external to said coil window toprovide easy alignment;

successively removing said other groups from the assembly, going fromthe innermost outward, and fixing each respective group on said coilsuch that the butt-joints are positioned internal to said coil window;

removing said single lamination from said clamped position andsuperposing it over said groups such that the butt-joint is positionedexternal to said coil window.

References Cited UNITED STATES PATENTS 3,204,210 8/1965 Duenke 336-217 X3,223,955 12/1965 Olsen et al 336-211 3,307,132 2/1967 Ellis 336--21l3,001,163 9/1961 Pfuntner et a1. 336-217 3,025,483 3/1962 Treanor336-217 X 3,341,941 9/1967 Olsen 29609 LARAMIE E. ASKIN, PrimaryExaminer.

DAVID A. TONE, Assistant Examiner.

